Again there is the counsel against discriminations between good and evil, since the original Mind transcends these:
Do not choose what is good, nor reject what is evil, but rather be free from purity and defilement. Then you will realize the emptiness of sin.11
This is not a preachment of values; rather it is the insight that there is a reality beyond our puny discriminations. If you can achieve this larger perspective, then good and evil become an inconsequential part of the larger flow of life.
His sermon then returns to the theme of the mind as the arbiter of reality, recalling the Void of Nagarjuna and pointing out that even the workings of the mind are ephemeral and possess no self-nature.
Thoughts perpetually change and cannot be grasped because they possess no self-nature. The Triple World [of desire, form, and beyond-form] is nothing more than one's mind. The multitudinous universe is nothing but the testimony of one Dharma [truth]. What are seen as forms are the reflections of the mind. The mind does not exist by itself; its existence is manifested through forms. . . . If you are aware of this mind, you will dress, eat, and act spontaneously in life as it transpires, and thereby cultivate your spiritual nature. There is nothing more that I can teach you.12
The essence of this teaching is that reality is, for us, merely what our mind says it is, and "enlightenment" or "becoming a Buddha" is merely coming to terms with ourselves and with this tricky mind that constantly devises our reality for us.
This credo is remembered most vividly in two anecdotes that were later enshrined in a famous collection of koans called the Wu-men Kuan (or Mumonkan in Japanese). In both of these anecdotes, Ma-tsu is asked, "What is Buddha?"—meaning what is the spirituality that all seek. In one he replied, "Mind is Buddha" (Mumonkan, Case 30), and in the other anecdote he said, "No mind, no Buddha" (Mumonkan, Case 33), which merely affirms that spirituality is in the mind, and for its realization one must realize the mind.13 In either instance he is merely following the earlier idea that there is no reality and thus no enlightenment outside the mind.
These two exchanges are part of a single anecdote of Ma-tsu recorded in the chronicles.
A monk asked why the Master maintained, "The Mind is the Buddha." The Master answered, "Because I want to stop the crying of a baby." The monk persisted, "When the crying has stopped, what is it then?" "Not Mind, not Buddha," was the answer. "How do you teach a man who does not uphold either of these?" The Master said, "I would tell him, 'Not things.' " The monk again questioned, "If you met a man free from attachment to all things, what would you tell him?" The Master replied, "I would let him experience the Great Tao."14